Roy's MAX Clothing Tutorials
There are six main means of giving your figures clothing. Which one will work best for you depends on what kind of clothing you are trying to model, what sort of clothing objects you have access to or can model, and whether you need to animate the figure versus rendering still frames. Each method has various pro's and con's, luckily you can mix and match each method on a single figure to get the effect you want with the least amount of frustration.

Method 1: Texture/Bump Maps

If you are going to model tight-fitting clothes and you need to animate your figure, texture maps are the way to go. Basically you paint the clothing you want you want (works perfect for tattoos as well) onto a bitmap with your favorite paint program. Use as large of a bitmap as your machine can handle for best results. Inside Poser, choose the Surface Materials... option and assign the bitmap as a texture map for your figure. Fortunately, Poser 3 comes with some nude texture maps you can start with as a template. There are places you can paint all of the pieces that make up the figure. You cannot move anything when editing a template. Unfortunately, the default texture maps are not large enough if you want to see your figure up close, so scale them up to say 1000x1000. You can make them any size you want as long as you don't change the aspect ratio. Another difficulty is that for some reason MC made the texture map from the back a different size than the front. This makes it difficult to get the seems where the two meet to match perfectly. Draw several horizontal test lines across the edges and work with Poser's display mode set to full texture mapping if you can. If you want to use a bump map, convert your texture map to grey-scale, make the skin a middle grey color and paint the area with the clothing white. Again, you must use very large bitmaps to get any kind of effect. One of the best features of this method is, you can redo the clothes whenever you want if you want to try different styles.
Pros: Very easy to do
Works very well with animation
Can change your mind at any time
Cons: Can only be used for tight-fitting clothes
Its hard to get the seams to match
Nipple and belly button geometry show through

Method 2: Props

Props work the way real clothing does. If you want a hat, you add a hat to your figure, Want a shirt? Add it as well. Props work well for only certain types of clothing, and are fairly difficult to build if you don't happen to have them.

To add a prop in Poser, you must a) have the prop on hand, b) import the .OBJ file, and c) double-click the prop to bring up its properties dialog and set the parent of the prop to be where the prop belongs (Eg. A hat goes on your head.)

Here's some more details on each part: a) You can buy props, download free props or build them yourself (hard to do, but if you can't find them anywhere else...) b) Yes, there is a bug in Poser 3 that sometimes causes rendering errors with .OBJ props, if you encounter this try getting the prop as a .DXF file. Poser also has a bug where the UV mapping coordinates for .OBJ files don't come in correctly. c) Always add your props very first thing, before you have posed your figure, otherwise they will not track properly.

There is a limitation that you cannot give a prop multiple parents, that is you can't make a long sleeve shirt with this method because if you parent the prop to the chest and bend the figure's arms, the shirt sleeves won't move with the arms.
Pros:Fairly easy, if you have the prop already made
Works fairly well with animation
Cons: Props can only have a single parent, they can't span multiple body parts
Several bugs exist with this method, all should be fixed in patch
Cannot easily switch props once you've started animating

Method 3: Replace Body Parts

Another way, very similar to the above, is to make props that can replace the body part which they would be parented to.

Method 4: Multiple Figures

Another way you might be able to get the clothing you desire is kind of tricky. Poser comes with several models that have clothes, there is a trick you can use to mix and match them. Say for example you want to make a topless construction worker in pants and boots. What you can do is this: Add two figures, one is the fully clothed version, the other is the naked version. Set them directly on top of each other. Select the clothed man's upper pieces and use their properties dialog to hide them. Repeat this process for the naked man's lower pieces. Unfortunately, you will have to do this one part at a time. You can use more than two figures if you have to, but you will have to pose each figure you add individually.
Pros:Simple to do
Cons: You must have a figure with the clothing you want
Doubles setup time for animated scenes
Cannot easily change

Method 5: Morph Targets

An advanced way to make clothing would be to make a morph target of the clothing you desire, add it to your figure and set its dial to be 1. Think of a morph target as a different version of a similar object. In fact, morph targets are used for all the facial animation inside Poser. They made a smiley face morph target for a happy face, then as you adjust the dial, it interpolates between the normal face and the smiley face. If you needed clothing that spanned multiple body parts you would just need multiple morph targets. However, this method is not going to be anyone unless MC or Zygote happens to come out with a CD of morph targets.
Pros: Would be simple, if morph targets were available
Would work well with animation
Cons: No morph targets are available
Beyond most peoples abilities to make any

Method 6: Special Model

Another way not likely to be used much is to just buy or build your own custom figure. Zygote will be happy to build you one, but I imagine their price is going to be prohibitive for the average user. You could build your own if you have a modeling package and the skills, but you will have to set up all of the joint parameters inside Poser yourself once its finished. This will be a lot of work.
Pros:Would be simple, if you could just buy one pre-built with joint parameters
Would work perfectly with animation
Cons: Much to much work to make your own
Cost prohibitive